1. Field of the Invention
This invention relates generally to Stirling engines and heat pumps and more particularly to improvements in free-piston, multi-cylinder Stirling engines and heat pumps arranged in an alpha configuration.
2. Description of the Related Art
Stirling machines have been known for nearly two centuries but in recent decades have been the subject of considerable development because of advantages they offer. In a Stirling machine, a working gas is confined in a working space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do work or to pump heat. Stirling machines cyclically shuttle a working gas between the compression space and the expansion space which are connected in fluid communication through an accepter, regenerator and rejecter. The shuttling is commonly done by pistons reciprocating in cylinders and cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and/or gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and/or gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in both spaces at any instant of time because they are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space then the machine is inclined to produce work and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm sink.
Stirling machines can therefore be designed to use the above principles to provide either (1) an engine having pistons driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space, or (2) a heat pump having pistons cyclically driven by a prime mover for pumping heat from the expansion space to the compression space. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used to generically include both Stirling engines and Stirling heat pumps.
Until 1965, Stirling machines were constructed as kinematically driven machines meaning that the pistons are connected to each other by a mechanical linkage, typically connecting rods and crankshafts. The free piston Stirling machine was then invented by William Beale. In the free piston Stirling machine, the pistons are not connected to a mechanical drive linkage. Free-piston Stirling machines are constructed as mechanical oscillators and one of its pistons, conventionally identified as a displacer, is driven by the working gas pressure variations in the machine. They offer numerous advantages including the control of their frequency and phase and their lack of a requirement for a seal between moving parts to prevent the mixing of working gas and lubricating oil.
Stirling machine have been developed in a variety of configurations. A common form of the modern Stirling engine is the alpha configuration, also referred to as the Rinia, Siemens or double acting arrangements. In the alpha configuration, there are at least two pistons in separate cylinders and the expansion space bounded by each piston is connected to a compression space bounded by another piston in another cylinder. These connections are arranged in a series loop connecting the expansion and compression spaces of multiple cylinders. The connection of each expansion space to the compression space associated with another piston typically includes, in series: (1) a heat exchanger for applying heat to the working gas, (2) a regenerator and (3) a heat exchanger for removing rejected heat from the working gas. Their expansion and compression spaces have been interconnected by identical length passages resulting in a box-four arrangement that is illustrated in FIG. 1. More specifically, FIG. 1 shows a conventional, alpha configured, box-four arrangement of four pistons 10 slidable in four parallel cylinders 12. An expansion space 14 of each cylinder 12 is connected to a compression space 16 of another cylinder 12 to form a series connected, closed loop. Each connection is through a series connected: (1) accepter heat exchanger A that accepts heat from an external source and transfers it to the working gas in the expansion space 14; (2) a regenerator R; and (3) a rejecter heat exchanger K that transfers heat rejected from the compression space 16 and rejects it to an external mass. The conventional art has configured these machines in this box-four arrangement in the kinematic versions of this machine. This arrangement is unduly restrictive by requiring four moving parts plus the attendant crank mechanisms and by requiring that the cylinders be set up at each corner of a square.
Generally, alpha Stirling machines have been constructed as kinematically driven machines. The phasing of the crankshaft throws have been such that the relative phasing between the pistons is always 90°. This has limited the power control at a given speed to mean pressure adjustment or stroke control.
William Beale suggested a free-piston, alpha configuration machine in 1976. However, as far as is known, no arrangements of multiple-cylinder, free-piston, Stirling machines have been disclosed other than the simple four cylinder one originally suggested by Beale. The advantages of the free-piston version of the alpha machine are the advantages that accrue to the free-piston arrangement, namely: no oil lubrication, no mechanism components, simple implementation of gas bearings, modulation by stroke adjustment and hermetic sealing of the machine against working gas leakage. The alpha arrangement has always been seen as an overly complicated implementation of the free-piston Stirling when compared to the conventional displacer-piston or beta configuration.
For completeness, the second Stirling configuration is the Beta Stirling configuration characterized by a displacer and piston in the same cylinder. The third is the gamma Stirling configuration characterized by locating the displacer and piston in different cylinders. The present invention deals with alpha configuration, free-piston Stirling machines.
The conventional layout of a single nth element of an alpha configured Stirling machine in free-piston mode is shown in FIG. 2. A piston 20 is matingly slidable in a cylinder 22 and bounds an expansion space 24 at it upper face 26. A piston rod 28 extends through a bearing 30 into connection with a spring 32 and a symbolic dashpot 34 to represent damping. The annular end face 36 of the piston 20 bounds a compression space 38. A compression space port 40 connects to the series connected heat exchangers and regenerator of another similar element and through them to the expansion space of another cylinder. A port 42 leads from the series connected heat exchangers 44 and 46 and regenerator 48 to the compression space of another cylinder. FIG. 2 represents only the Stirling machine. A load is also connected to the piston rod 28 in the case of a Stirling engine and a prime mover is connected to the piston rod 28 in the case of a Stirling heat pump. The arrows leading from the piston and pointing upwardly in FIG. 2, as well as similar arrows in other Figures, designate the directional convention for positive piston displacement or stroke.
It is clear and generally understood that the alpha machines may be compounded in the multi-piston forms shown in FIG. 3 to have up to five cylinders connected together as described, although there could be more. Alongside each multi-piston example of FIG. 3 is a phasor diagram illustrating the cyclic piston motion and the cyclic expansion and compression space volumes of the associated example. The phase angle between the expansion space volume and the compression space volume in a Stirling machine is of critical importance because power and efficiency are a function of this phase angle. In early alpha Stirling machines, the volume phase angle was fixed at 90° by the orientation of the cylinders and connection of the pistons through connecting rods to a crank. However, for any Stirling machine, the preferred volume phase angle is within the range of 90° to 140°. This can be seen with reference to FIG. 14 which shows graphs of power and efficiency as a function of volume phase angle. It is desirable to operate the Stirling machine near the peaks of both the efficiency graph and the power graph. Lower and higher volume phase angles result in compromised efficiency and power. The poorer performance at the lower volume phase angles is due to high flow losses, high hysteresis losses and poor capacity (power or heat lift) per unit volume. The most favorable phase angle is generally around 120°. Volume phase angle is a function of the relationships of the expansion space and compression space volume phases to piston motion. Those relationships are a function of the machine structures and therefore the volume phase angle between the expansion space volume and a connected compression space volume is a function of machine structure.
In the phasor diagrams of FIG. 3, the volume phase angle α is shown in each case for a single set of expansion and compression space volume variations and would be the same for the other sets in the same example. By convention, α is the angle by which the expansion space volume leads the compression space volume. In the case of the conventional construction illustrated in FIGS. 1–3, the expansion space volume variations are in anti-phase with the piston motions while the compression space volume variations are in phase with the piston motions. As shown in the phasor diagrams of FIG. 3, a three-cylinder version of the conventional alpha compounding would have a poor volume phase angle at 60°. A four cylinder version would have a volume phase angle of 90° and a five cylinder version would have a volume phase angle of 108°. In order to obtain a volume phase angle of 120°, with the conventional alpha configuration, six cylinders would be needed.
In addition to the desirability of attaining a highly efficient volume phase angle, it is also desirable to reduce the number of component parts required for a Stirling machine and to minimize its weight and volume. Each beta Stirling configuration has two essential moving parts and in most cases also needs to be balanced, for example by a resonant balance mass that is attached to the casing. The alpha configuration is seen to require four essential moving parts, four pistons, in order to have an acceptable phase angle. A secondary difficulty of the alpha free-piston configuration is that it requires four linear alternators (or motors, in the case of a heat pump) because one is needed for each piston. Linear alternators have been somewhat bulky compared to their rotating counterparts and this has led to a feeling in the art that the alpha machine may be bulky and the cylinders inconveniently far from each other leading to a heavy machine. The balancing of a conventional alpha configuration is also not trivial and does not seem to have been addressed in the open literature.
An ideal solution to the alpha free-piston complexity would be a device that: improves the power to weight ratio of free-piston Stirling machinery without additional complication and thereby reduces the cost of the device; reduces the number of moving parts; provides a compact means for connecting a load to the machine so that the cylinders are not spaced too far apart; and provides a simple means of balance or of reducing the out of balance forces. The proposed invention appears to reduce or solve these problems in a simple and practical manner.